This invention relates to an improved ignition plug for an internal combustion engine such as a gasoline engine and more particularly, to such an ignition plug which is capable of effectively discharging residual gas which often remains in the engine cylinder after one cycle operation of the engine before the engine is fired for the next cycle operation.
Such residual gas in the cylinder of the internal combustion engine generally tends to reduce the average effective pressure and charging efficiency of the engine and substantially increase the temperature of the cylinder of the engine and thus, the temperature and weight of the residual gas has a great effect on the performance of the engine.
Especially, it has been found that the pressure of a portion of the combustion gas within the engine cylinder is reduced to a value lower than atmospheric pressure on the air-intake, explosion and scavenging strokes in one cycle operation of the engine. Such pressure drop phenomenon occurs in various types of engines such as gasoline engines, Diesel engines and rotary engines and the cause of the gas pressure drop is considered as the result of the reduction in pressure of the burnt combustion gas at the time of propagation of flames.
In order to overcome the difficulties, various attempts have been made by blowing secondary air into the engine cylinder so as to reduce the weight and temperature of the residual gas in the engine cylinder. As one of these attempts, the applicant has developed an ignition plug which concurrently has the function as a check valve. The ignition plug generally comprises a hollow anode member having a center through opening, a center electrode extending through the center opening in co-axial therewith and having an air intake passage which communicates the interior of the engine cylinder with atmosphere and a check valve provided in the air intake passage for the prevention of reverse flow of the gas within the cylinder.
The ignition plug is easily adaptable to the above-mentioned secondary air introduction method without substantially modifying the principal components of the engine in which the ignition plug is incorporated and has been expected that the ignition plug would operate effectively.
However, in the ignition plug referred to herein-above, since the center electrode having a very small diameter is provided with the air intake passage and check valve, the ignition plug is not expected to have sufficient mechanical strength and service life. And since the construction of the ignition plug is such that a valve chamber having a tapered seat face is formed on the inner surface of the air intake passage in the center electrode and a spherical valve member is received in the valve chamber and normally urged against the tapered seat face by means of a compressed spring, the ignition plug can not provide a satisfactory air-tight seal. In addition, the check valve frequently fails to open and close in proper timing relationship to the strokes in one cycle operation of the engine and instead the gaseous mixture flows in the reverse direction within the cylinder to cause an abnormal explosion in the cylinder resulting in premature wear of the check valve. Thus, the ignition plug of the above-mentioned type can not be utilized to full advantage inherent in the external air introduction method and does not provide its expected effects. Such being the situation, the ignition plug has not been practically employed.